1. Field of the Invention
The invention relates to a method of assigning a pertinent (permanent) indication with respect to color behavior to a picture element to be imaged with a color reproduction system and to the color reproduction system.
2. Discussion of Related Art
In the reproduction of colored information on information supports in a color reproduction system, a problem arises as to the way in which pastel hues can be reproduced in a perceptually acceptable and yet economically viable way. As the degree of saturation of a pastel color further decreases in proportion with respect to a picture element to be reproduced, at some time a point will be reached at which it is perceptually no longer logical to reproduce as such the color information associated with such a pastel color but, instead, as grey information. This also has the advantage that only a black reproduction unit is required to operate in the color reproduction system, and not one or more other color printing units as well, and this means less expensive operation of the system and reduced wear therein.
The boundary line to be drawn in a color space to obtain a transition between achromatic and chromatic reproduction of picture information should first be determined on perceptual and technological grounds. This problem has already been addressed in U.S. Pat. No. 2,918,523. However, in this case, for the sake of simplicity, achromatic reproduction of pastel colors has been selected, although this does not produce a satisfactory result. A picture element which perceptually will be considered as colorless and which is in the neighborhood of the boundary may, as a result of scanner imperfections (such as scanner noise), be detected, not as colorless, but as color-bearing, or vice versa. This occurs particularly in the case of picture elements situated close to the transition from the colorless area to the color-bearing area in the color space. An additional factor to be taken into account is that the scanner noise increases in proportion as the intensity of the picture element increases. It is therefore logical to distinguish in a color space a kind of uncertainty area which includes parts situated on either side of the boundary and in which, because of the possible occurrence of scanner noise, a picture element occurring there may not automatically be assigned a color behavior or a colorless behavior (colorlessness) on the basis of its parameter values, but only a provisional indication concerning an undefined color behavior. Each picture element occurring in that uncertainty area must then be subjected to a further and more detailed test to arrive at a more pertinent if not permanent, indication or label with respect to color-bearing or colorless behavior. Distinguishing between picture elements having a pertinent indication of color behavior, picture elements having a pertinent indication of colorless behavior, and picture elements having a provisional indication concerning an undefined color behavior, is already known from EP-A-0395032. In the system described therein, however, the entire area of picture elements with a provisional indication is sacrificed for the elimination of register errors, a picture element becoming colorless in the case of a text picture, and having color if a color picture is involved. The elimination of scanner noise errors is therefore not addressed here. Distinguishing between picture elements having a pertinent indication of color behavior, picture elements having a pertinent indication of colorless behavior, and picture elements having a provisional indication concerning an undefined color behavior, is also known from EP-A-0363146 and EP-A-0411911, in which use is made of certain comparison criteria which, however, are tested in a space derived from the RGB color space and defined by the maximum and minimum color component values with respect to a picture element in such color space. A method of this kind requires considerable computing work. The maximum and minimum color component value must first be determined with respect to each picture element and then a set of mathematical inequality functions must be examined to define the area that picture element forms a part of. For a picture element having a provisional indication concerning an undefined color behavior, a pertinent indication with respect to a color-bearing or colorless behavior is then determined in this patent publication by reference to an environmental analysis. The environmental analysis to be performed under these conditions means comparing the number of pertinent indications relating to color behavior in an environment (e.g. a 5.times. 5 picture element space) around the associated picture element, with the number of pertinent indications relating to a colorless behavior, the result thereof then indicating what type of pertinent indication must be assigned to the associated picture element. An environmental analysis of this kind, however, is intended to trace and eliminate register errors but not to eliminate scanner noise errors. The said environmental analysis also has the disadvantage that it produces unreliable results in an area of soft pastel hues, since relatively few or no pertinent indications with respect to a color behavior or colorless behavior occur in such an area. In an area of soft pastel hues this can result in rapid variations in the type of pertinent indications to be assigned to successive picture elements, although they may not be reproduced differently according to the original. Also, in an area having soft pastel hues in which there are few picture elements with a pertinent indication, a small majority of one of the types of pertinent indications may, during the performance of the environmental analyses, cause the excess proportion of the provisional indications to be replaced by pertinent indications of the majority type, resulting in an incorrect color impression.